A brush DC (direct current) motor adopts an electric brush (herein referred to as a brush) to cooperate with a commutator to realize continuous conduction and current commutation of a power supply circuit of the motor. A front end surface of the conventional brush is a planar surface when in an initial state. However, the commutator is cylindrical, and a contact between a newly assembled brush and the commutator is generally a line contact, which is unstable. As a result, an irregular vibration of the brush is generated during rotation of the commutator, thus producing mechanical noise and sparks. In addition, during rotation of the commutator, when adjacent commutator segments are short circuited by one brush, the current of the motor fluctuates significantly and thus forms a ripple current. In the prior art, measuring a rotational speed of the motor rotor using the ripple current has been developed, which reduces the cost of a speed sensor by eliminating the magnetic sensor and the magnet ring. However, the unstable contact between the brush and the commutator increases or magnifies the irregularity of the ripple current, which leads to inaccurate readings or a complete failure of measuring the rotational speed by using the ripple current. This is particularly relevant to gear motor assemblies, also known as motor actuators or actuators, due to their need for accurate position determination in many applications.